Down feather and synthetic fiberfill are the two most commonly used materials one will find in any sleeping bag today. Each of these has their own pros and cons. Down is very warm and light weight but is also very expensive. In general, the thermal performance in a down product is directly proportional to the amount of down used (fill weight), so the construction in a down product is more to improve the user experience rather than to improve thermal efficiency. Synthetic fiberfill exists in the form of a padding which is made up of polyester fibers. Different proportions of the different fiber type results in various types of fiberfill padding available in the market today. Similar to down product, fiberfill product's performance is positively related to fill weight increase. However, there is a fundamental difference in construction between a down sleeping bag and a fiberfill sleeping bag.
Down behaves like a fluid, so the process of making a down sleeping bag is to create a compartment of space, and then fill it with the down material. The architecture/construction of the compartment, other than the necessary volume adjustment, is not a major determining factor to the overall thermal performance of the product. In a fiberfill sleeping bag, the fill weight adjustment is usually attained by putting together multiple layering of the fiberfill padding together in which the padding itself is also part of the overall product construction/structure. An effective construction technique plays a very important role in maximizing the overall thermal performance efficiency in a fiberfill sleeping bag.
There are various types of fiberfill construction: offset layering, shingle construction, anchored flexure system etc. Each of these has its respective benefits/shortcomings.
A common characteristic among all of the constructions above is that the quilting of the product (sleeping bag) always crosses horizontally (from side to side across a user's body—FIG. 1). Regardless of sizes, weight, constructions, brands, essentially all fiberfill sleeping bags are with horizontal quilting. The reasons for such setup are mostly due to better yield (consumption) and loft enhancement. Since the fiberfill padding is also part of the sleeping bag construction, the limiting factor is actually tied to the yield of fabric. Horizontal quilting/cuts give the best fabric yield in a sleeping bag and thus making it a natural fit in construction.
Loft is referring to how thick the layering is in a cross-section. The thicker the loft, the greater the distance separating the user from the outer environment which reduces the body heat escaping to the cold air outside. Loft is not only part of the specification in a sleeping bag, but also brings a very important visual appeal to the general consumer.
From a thermal performance perspective, horizontal quilting and loft are, however, somewhat mutually exclusive. Horizontal quilting requires the orientation of the fiberfill padding/cut to go from side to side. In such orientation, the loft expectation is the sum of the thickness of each padding layer. In simple math term, 1+1 will always be 2. An anchored flexure system enhances system loft by bending one of the padding layers. By taking advantage of the rebound of the fiberfill padding, an anchored flexure system is able to add an X factor to the loft (2+X). The down side to this is that the X factor can vary. Its effectiveness is determined by the type of fiberfill padding used, thus making the loft enhancement somewhat inconsistent.
The introduction of vertical quilting through the use of Vertical Inner Valve (U.S. Pat. No. 7,818,834 B2) has been very well received in the outdoor industry. Other than the visual enhancement, vertical quilting/baffling actually brings a very meaningful improvement to the user experience in a sleeping bag. It is very natural to think that the same technique can be applied to fiberfill synthetic sleeping bag as well. As established earlier, although both down-filled and synthetic fiberfill-filled share the same goal, the approach from manufacturing perspective is totally different from one another. If the goal is just to make the product look different, anyone can just simply turn any existing horizontal quilting into a vertical form by rotating it 90 degree. There is really no novelty to do this at all as the thermal performance is still limited by the nature (thickness) of the padding layer. So the challenge is how to improve the overall system performance in a vertically quilted sleeping bag. This forms the basis of the tubic construction.